Effects of pavements on established urban trees: Growth, physiology, ecosystem services and disservices

This research investigated the effects of impermeable pavements, permeable pavers, and permeable concrete on growth, physiology, and ecosystem services provisioning of established Celtis australis and Fraxinus ornus plants over a 5-year-period. In March 2012, 48 trees were planted into 1 m2 planting pits according to a randomized block design with 6 blocks, and randomly assigned to one of the four treatments: 1) impermeable pavement; 2) permeable pavers; 3) permeable concrete; 4) bare soil as unpaved control. Trees were fully established in December 2015, when this experiment begun. Above-ground tree growth, below-ground growth, tree physiology, soil physical and chemical traits, and root conflicts with pavements were monitored until December 2020. Pavements did not affect soil chemical traits, but impermeable pavements reduced soil moisture, compared to other treatments, with larger differences observed in winter and spring than in summer and fall. No evidence was found that pavements promoted drought stress in established trees, nor pavements depressed above-ground growth, although root morphology was influenced by pavements, likely due to elevated soil CO2 beneath pavements with lower gas diffusivity. CO2 assimilation, carbon storage, and cooling by transpiration 9 years after planting were not affected by pavements, but Celtis provided far more ecosystem services than Fraxinus. Permeable concrete was effective in mitigating the urban heat island effect by promoting higher evaporation than both impermeable and permeable pavers, but their durability should be improved before large-scale use can be recommended.